Hydrogenative Cycloisomerization and Sigmatropic Rearrangement Reactions of Cationic Ruthenium Carbenes Formed by Catalytic Alkyne gem‐Hydrogenation

Abstract gem‐Hydrogenation of propargyl alcohol derivatives with [CpXRu(MeCN)3]PF6 (CpX=substituted cyclopentadienyl) as catalysts affords cationic pianostool ruthenium carbene complexes which are so electrophilic that they attack a tethered olefin to furnish cyclopentene products; cyclopropanation or metathesis do not compete with this novel transformation. If the transient carbenes carry appropriate propargylic substituents, however, they engage in ([2,3]‐sigmatropic) rearrangements to give enol esters (carbonates, carbamates, sulfonates) or alkenyl halides. Both pathways are unprecedented in the vast hydrogenation literature. The proposed mechanistic scenarios are in line with labeling experiments and spectroscopic data; most notably, PHIP NMR spectroscopy (PHIP=parahydrogen induced polarization) provides compelling evidence that the reactions are indeed triggered by highly unorthodox gem‐hydrogenation events.

S6 Data Processing. The raw data was then imported into Excel and plots of log(conc.) vs. time were prepared. The resulting linear graphs indicate that the decomposition of the carbene complexes follow first-order kinetics. The slope of the obtained lines gives rise to the kobs values of the individual decomposition reaction, which reflect the different thermodynamic stabilities. Supporting Information

S7
The kobs values were also plotted against one-electron oxidation potentials (E1/2) of the corresponding [Cp X Ru(MeCN)3]PF6 complexes measured by cyclic voltametry. 7 The good correlation (R 2 =0.98) between these two variables indicates that the electronic properties of the Cp x ligand is the prime factor that influences their thermodynamic stability.
Supporting Information S18 Figure S1. Top: Comparison of the 1 H NMR spectrum of authentic 12 8  solution was transferred into a pressure NMR tube (5 mm medium wall precision pressure/vacuum valve NMR sample tube, Wilmad-LabGlass), which was tightly closed and then taken out of the glovebox. The tube was connected to the parahydrogen (pH2) generator and all tubings were evacuated and backfilled with pH2 three times.The pressure was then increased to 5 bar and the valve was opened to fill the tube with pH2 to a total pressure of ca. 6 bar [the insert of the monitor of the pH2 generator shows the pressure in barg (pressure above atmosphere)]. After closing the valve, the tube was shaken and inserted into the NMR magnet.
We have previously communicated that electron-deficient Cp x Ru complexes are particularly good catalysts for gem-hydrogenation. 7  Supporting Information S20 Figure S2. Methylene region of the PHIP NMR and OPSY NMR spectra of complex 14a formed from compound 13a and catalyst C5; the upper spectrum was obtained after a /4 pulse (PASADENA signals), the lower spectrum with an OPSY filter prior to acquisition of the FID Figure S3. Methylene region of the PHIP NMR and OPSY NMR spectra of complex 14b formed from compound 13a and catalyst C3; the upper spectrum was obtained after a /4 pulse (PASADENA signals), the lower spectrum with an OPSY filter prior to acquisition of the FID

Deuterium Labelling Study
Analyses of the degree of deuteration of the starting material and the product are provided below.
They show an essentially quantitative migration of the deuterium label from the internal vinylic position to the terminal site.

Ligand Effects in Hydrogenative Rearrangements
Because gem-hydrogenation of alkynes is mechanistically intertwined with trans-hydrogenation,

Hydrogenative Rearrangement and Cycloisomerization Reactions: PHIP NMR Studies.
At the outset of this project, we surmised that hydrogenative rearrangements could proceed via two distinct pathways. On the one hand, gem-hydrogenation could generate competent ruthenium carbenes. Alternatively, π-acid activation of the alkyne would entail nucleophilic attack of the propargylic substituent to yield a vinyl metal species, which could further evolve into a vinyl carbene; hydrogenolytic cleavage would then release the product and close the catalytic cycle. It is important to note that a closely related reactivity mode was observed in one of our previous publications. 9 In order to clarify which pathway is operative, PHIP NMR was again used as the analytical tool.

tert-Butyl(((1s,4s)-1-ethynyl-4-(trifluoromethyl)cyclohexyl)oxy)dimethylsilane (S15).
Ethynylmagnesium bromide (0.5 M in THF, 12.9 mL) was added dropwise to a solution of 4-trifluoromethylcyclohexanone (830 mg, 4.99 mmol) in THF (15 mL) at 78 °C. The mixture was stirred for 1 h at 78 °C and for 2 h at ambient temperature. sat. NH4Cl solution was introduced and the phases were separated. The aqueous phase was extracted with tert-butyl methyl ether (3 x 20 mL) and the combined organic layers were washed with brine and dried over Na2SO4. All volatile materials were removed in vacuo and the residue was directly used in the next step without further purification.
2,6-Lutidine (0.26 mL, 2.28 mmol) and tert-butyldimethylsilyl triflate (0.39 mL, 1.71 mmol) were added to a solution of the crude alcohol (220 mg, 1.14 mmol) in methylene chloride (10 mL) at 78°C. The solution was allowed to reach ambient temperature within 15 h. Water (10 mL) was added and the phases were separated. The aqueous phase was extracted with methylene chloride (3 x 10 mL) and the combined organic layers were washed with brine and dried over Na2SO4. All volatile materials were removed in vacuo and the residue was purified by flash chromatography (SiO2, pentane:tert-butyl methyl ether, 100:1) to provide the title compound as a colorless liquid (62 mg, 18%  were removed in vacuo and the residue was used in the next step without purification.
The solution was stirred at reflux temperature for 15 h before it was cooled to room temperature and diluted with tert-butyl methyl ether (10 mL). The suspension was filtered through a short plug of Celite which was carefully rinsed with tert-butyl methyl ether (20 mL). The combined filtrates were evaporated in vacuo and the residue was purified by flash chromatography (silica, pentane/tert-butyl methyl ether, 10:1) to provide the title compound as a colorless oil (298 mg, 57%  with hydrogen for 2 min using a hydrogen-filled balloon and an outlet cannula. The outlet cannula was removed and the reaction mixture was vigorously stirred (1200 rpm) under a hydrogen atmosphere. After 16 h, the flask was vented, the solution was diluted with pentane (4.0 mL) and filtered through a plug of silica, which was rinsed with pentane/tert-butyl methyl ether (5:1, 12 mL).